Abstract: A survey was conducted with the square intercept method on species and distribution of weeds in habitats where Alternanthera philoxeroides (Mart.)Griseb.grew in Nanjing.Principal component analysis (PCA) of the collected data showed that there were 101 weed species belonging to 30 families recorded in autumn in the region,and that the most important factors affecting the distribution of weed species were soil humidity and human activities.According to the two factors,all the samples were divided into three groups.Group I,including the samples collected from roadsides,open spaces beside the fields and shores of rivers,ditches and lakes,was dominated with weed species such as Digitaria sanguinalis,Setaria viridis and Eleusine indica.Group Ⅱ,consisting of the samples located on ridges and furrows in the fields,was dominated with weed species,like Veronica persica,Conyza bonariensis and Acalypha australis.And Group III of the samples gathered from orchards and kaleyards,which lacked management and moisture,was dominated with species,like Digitaria sanguinalis,Portulaca oleracea and Cyperus rotundus.The analysis of relationship between the importance value of Alligator weed and species diversity of the samples showed that when the former reaches 1.5,the latter decreased with the rise of the former,demonstrating a significant correlation between them,which suggests that the invasion of Alligator weed is harmful to biodiversity.

Key words: Alternanthera philoxeroides, weed distribution, principal component analysis, biodiversity